1. Field of the Invention
The present invention relates to a steering system in which an operation of toe angle of rear wheels is controlled based on a turning angle of front wheels and a vehicle speed, and particularly to a steering system in combination with an electric power steering device which assists steering wheel turn of the front wheels.
2. Description of the Related Art
An electric power steering device is a device in which an electric motor generates an auxiliary torque in accordance with a magnitude of a steering torque, and the auxiliary torque is transmitted to a steering unit, to thereby reduce a steering effort required by a driver for steering. There is disclosed a technique in which a base current (assist torque) determined by a steering torque and a vehicle speed is compensated by inertia and damping (viscosity) in the steering unit, and the compensated current is used as a target current for controlling an electric motor (see Japanese patent application unexamined publications No. 2002-59855 (FIG. 2) and No. 2000-177615 (FIG. 2)).
As an electric motor for such an electric power steering device, a brushless motor is disclosed in Japanese patent application unexamined publication No. 2004-322814 (or corresponding U.S. Pat. No. 6,926,299) (FIGS. 2 and 3).
Also in Japanese patent application examined publication H6-47388 (FIG. 2), there is disclosed an all-wheel independent steering device in which operation of all running wheels are individually controlled based on an operation angle of steering wheels and a vehicle speed.
In Japanese patent application unexamined publication No. 2002-59855, properties including a base current, damping and inertia are computed using a base table, a damper table and an inertia table which substantially has a differential property, respectively. Herein, setting of each table, which includes functions of steering torque, vehicle speed and electric motor angular velocity, will be discussed. The base table is set in such a manner that a driver is provided with road information and a steady responsive feeling from a steering torque, in accordance with an increase in the vehicle speed, and thus it is required that a gain be made lower when the vehicle speed is higher, and that a dead zone is set larger for giving a larger manual steering zone. The base table is also set so as to give an excellent steering feeling, and therefore, it is required that a response lag, which may otherwise be caused by electric motor inertia, viscosity or the like, be reduced by using the inertia table.
The inertia table is set so as to improve vehicle properties by improving response of the steering wheel and also convergence of the steering wheel position, by cooperating with a steering damper effect. Therefore, it is required that the inertia table be substantially provided with a differential property based on the steering torque, to thereby increase or decrease an assist on the electric motor in accordance with the changed portion of the steering torque, i.e., a rotational acceleration (steering rotational acceleration) of the electric motor.
However, in a case of a steering system in which a toe angle changer which controls an operation of a toe angle of rear wheel, based on an operation angle of a steering wheel or a turning angle of front wheel as well as a vehicle speed, is combined with an electric power steering device, a responsive feeling from a steering torque given by the electric power steering device of the vehicle having the above-mentioned steering system may bring discomfort to a driver who is used to a responsive feeling from the steering torque given by an electric power steering device of a vehicle that has only a front wheel steering function.
In addition, even when the toe angle changer is in an abnormal state (for example, the toe angle of the rear wheel is locked) or when the steering of the front wheel is assisted by the electric power steering device, the driver may not feel any anomaly in the responsive feeling from the steering torque, and may keep operating the steering wheel in the same manner.
Therefore, it would be desirable to provide a steering system that solves the above-mentioned problems.